<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .box {
            width: 200px;
            height: 200px;
            display: flex;
            /* align-items: center; */
            background-color: aqua;
            /* justify-content: center; */
        }

        .content {
            width: 150px;
            height: 100px;
            background-color: red;
        }

        .content2 {
            flex-grow: 1;
            flex-shrink: 1;
            flex-basis: 100px;
            background-color: black;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="content"></div>
        <div class="content2"></div>
    </div>
</body>

</html>